1. Field of the Invention
The present invention is concerned with the electrolytic production of chlorine dioxide from chlorite ions. More particularly, the present invention relates to the electrochemical process and the electrolytic cell structure used to manufacture a high purity aqueous chlorine dioxide solution from a dilute aqueous alkali metal chlorite solution.
2. Description of the Art
It is known to produce chlorine dioxide electrolytically by the electro-oxidation of chlorite ions.
U.S. Pat. No. 2,163,793 describes an electrochemical chlorine dioxide generating process in which an aqueous solution of alkali metal chlorite and alkali metal chloride is electrolyzed in an electrolytic cell equipped with a porous diaphragm separating the anode and the cathode compartments.
British Patent No. 714,828 describes a process for the production of chlorine dioxide by electrolyzing an aqueous solution containing chlorite and a water-soluble salt of an inorganic oxy-acid other than sulfuric acid.
U.S. Pat. No. 2,717,237 discloses a method for producing chlorine dioxide by electrolysis of chlorite in the presence of a water-soluble alkali metal sulfate (e.g., sodium sulfate).
Japanese Patent Publication 81-158883, published Dec. 7, 1981, describes an electrolytic process for producing chlorine dioxide by electrolysis of chlorite in which the electrolyzed solution, at a pH of 2 or less, is fed to a stripping tank where air is introduced to recover the chlorine dioxide.
U.S. Pat. No. 4,542,008 describes an electrolytic process for chlorine dioxide production in which the sodium chlorite concentration of the solution leaving the anode compartment is measured by means of a photometric cell.
Published PCT International Patent Application WO 91/09158 and the corresponding U.S. Pat. No. 5,106,465 disclose a method of producing chlorine dioxide from alkali metal chlorite in an ion exchange compartment of a multi-compartment cell in which hydrogen ions generated in the anode compartment enter the ion exchange compartment through a cation exchange membrane, causing chlorite ion decomposition and forming chlorine dioxide.
PCT Published International Patent Application WO 94/26670 discloses a method of producing chlorine dioxide from sodium chlorite in which the gaseous product along with the water vapor is removed from the electrolyzed solution by means of a microporous, hydrophobic gas membrane. By removing water at the rate of its input to the anolyte, a continuous, environmentally innocuous operation with no undesired effluent can be effected.
While all the above mentioned patents and patent applications require the recirculation of the electrolyzed solution, PCT Published International Patent Application WO 91/09990 and its related U.S. Pat. Nos. (5,041,196, 5,084,149, 5,158,658, 5,298,280 and 5,294,319) teach an electrochemical process for producing chlorine dioxide from a dilute alkali metal chlorite solution in a single pass mode (i.e., with no recirculation of the anolyte) using a porous, high surface area anode. The product solution, in addition to chlorine dioxide, may also contain unconverted chlorite as well as undesired by-products, resulting from inefficiencies, such as chlorate or chloride ions.
The relative simplicity of the concept disclosed in WO 91/09990 and its related U.S. Patents makes it economically attractive. However, the presence of unconverted chlorite and undesired by-products in the product stream may preclude its use in many applications.
Therefore, there is a need for a chlorine dioxide generation process based on single pass mode with no recirculation of the anolyte wherein there is a high efficiency conversion of chlorite ions to chlorine dioxide per pass while minimizing the formation of undesired by-products.